1. Field of the Invention
The present invention relates to a device for detecting a synchronizing signal from a digital TV signal.
2. Background of the Related Art
A digital TV broadcasting system adopted by the ATSC(Advanced Television Systems Committee) in general uses a terrestrial VSB(Vestigial Side Band) transmission system. In the digital TV broadcasting system, a video data and a synchronizing signal are transmitted, separately. And, on a receiver side, a position of the synchronizing signal should be accurately detected from the video data. As the synchronizing signal indicates agreed sections from other sections, it is very important to detect the position of the synchronizing signal, accurately. In other word, because the synchronizing signal is a reference of time in processing signals of other units in the receiver, an accurate detection of the synchronizing signal at a data reception terminal is very important. In a digital broadcasting signal, a pattern of the synchronizing signal is in general inserted in the video data so that the synchronizing signal can be detected on the receiver side. Referring to FIG. 1 illustrating a related art VSB transmission data, in the VSB transmission system, a data stream(or bit stream) is used, in which particular synchronizing signal patterns are inserted at fixed intervals for easy detection of the synchronizing signal. In this instance, one section of data stream has 832 symbols of successive data segments, including four symbols of synchronizing section and 828 symbols of data section. The synchronizing section has two levels of signals of `-1` or `+1`, and the data section has eight levels of signals.
A related art device for detecting a synchronizing signal from a VSB transmission signal, a digital broadcasting signal, will be explained with reference to the attached drawings. FIG. 1 illustrates a waveform showing a related art data structure, FIG. 2 illustrates a block diagram showing a related art device for detecting a synchronizing signal from a VSB transmission signal, and FIGS. 3a-3c illustrate waveforms showing an example of detecting the synchronizing signal using the device shown in FIG. 2.
Referring to FIG. 2, the related art device for detecting a synchronizing signal from a VSB transmission signal is provided with a correlator 11 for subjecting a VSB transmission signal received in a form of data stream and a synchronizing signal pattern set in itself to correlation operation, a storage 12 for storing an output from the correlator 11, an adder 13 for adding a value stored in the storage 12 to an output from the correlator 11, a limiter 14 for limiting an output from the adder 13 not to exceed a given plus level, and a confidence check counter 15 for detecting a signal from outputs of the limiter 14 higher than the given plus level as a synchronizing signal, and removing the synchronizing signal which is detected by mistake momentarily.
The operation of the aforementioned related art device for detecting a synchronizing signal will be explained.
Upon reception of the VSB transmission signal in a form of bit stream, the correlator 11 subjects the signal and a synchronizing signal pattern set therein to correlation operation while shifting the VSB transmission signal by one symbol unit, and forwards results of the operations in succession. The correlation operation is one in which the synchronizing signal patterns of "+1, +1, -1, +1" are respectively multiplied to corresponding shifted data and results of the operation are added. If the data shifted presently is a synchronizing signal pattern, when the shifted synchronizing signal pattern and the synchronizing pattern stored in itself are subjected to a correlation operation, a resultant of the operation will be four which is a maximum value. That is, if the synchronizing signal pattern set in itself is "+1, -1, -1, +1", the resultant of the operation will be the maximum value "4" because the shifted synchronizing signal pattern will also be the same "+1, -1, -1, +1". On the other hand, since it is highly probable that the shifted synchronizing signal pattern has a symbol different from the preset synchronizing signal pattern in signal sections other than the synchronizing signal section, the resultants of the correlation operations will be below "4". Consequently, a resultant of the correlation operation obtained in an interval at which the synchronizing signal section in the bit stream and the section of the preset synchronizing signal pattern are met provides a signal higher than a level of the signal which is provided as a resultant of the correlation operation obtained in an interval at which the data section in the bit stream and the section of the preset synchronizing signal pattern are met. As shown in FIG. 1, the storage 12 has 832 memories in view of the repetition of the synchronizing signal at every 832 symbols. The adder 13 adds a value stored in the storage 12 and an output from the correlator 11 to update a value in the storage 12. In this instance, if the value in the storage 12 is updated as the output from the correlator 11 is added thereto by the adder 13, the value in the storage 12 corresponding to the synchronizing signal section will keep increasing while an average of the values in the storage 12 corresponding to the data section other than the synchronizing signal section will keep approaching to `0` because patterns are random, i.e., positive(+) outputs and negative(-) outputs are added by the adder 13 in the updating of the value is the storage 12, which results are shown in FIG. 3a. However, if these operations are kept repeating, peak values in FIG. 3a are kept greater until the peak values are diverged, thereby causing a malfunction of the synchronizing signal detecting circuit. Therefore, the limiter 14 limits a sum of the output from the correlator 11 via the adder 13 and the output from the storage 12 to be greater than the given plus level. Then, the confidence check counter 15 detects a position where one of the outputs from the limiter 14 is greater than the given level and makes a confidence check for reducing detection error caused by momentary very great noise, to detect accurate positions of the synchronizing signals as shown in FIG. 3a, finally.
In the meantime, as shown in FIG. 3b, when an external noise is occurred at a position spaced from the synchronizing signal due to ghost or others during reception of data, since the noise is at a level lower than the detected synchronizing signal theoretically, the external noise gives no great influence to the synchronizing signal detection. However, as shown in FIG. 3c, in the related art, because the synchronizing signal is limited to the give level by the limiter 14, distinguishing the synchronizing signal from the noise signal is not possible, practicably. Thus, as the limiting operation is made for preventing divergence of the output, the related art device for detecting a synchronizing signal from a VSB transmission signal has a problem in that a wrong synchronizing signal may be detected depending on the limiting level when a noise of a preset level is occurred. In other word, the use of limiter for preventing divergence of an accumulation loop may limit a synchronizing signal recovery performance depending on a limiting value used when a ghost signal is in a transmission channel.